The SR5 term was in use for many years before it ever showed up on the trucks. It only had a meaning on the very first car it showed up on, which I think was a Corona model, but it might be earlier.

Anyhow, for many decades now it's just a gimic badge on the truck you get to pay extra for. Generally some fluffy trim stuff comes with the badge. There is zero performance enhancement associated with it.

'97 T-100 SR5
'86 Toyota's, the variety pack (all gone)